Which of the following statements are true?

a. As load increases armature current decreases, regardless of excitation.

b. For under and over excited motors, power factor tends to approach zero with increase in load

c. With normal excitation when load is increased, change in armature current is greater than in power factor which tends to become increasingly leading.

d. Both with under and over excitation, change in power factor is greater than in armature current with increase in load.